1. Field of the Invention
This invention relates to an imagine pyrometer and a method for simultaneously determining surface temperature distributions and images of remote objects.
2. Description of Related Art
In machine vision and automatic manufacturing, high-resolution temperature maps and visible imagery must simultaneously be acquired that need to be in perfect geometric registration. In such applications, the readout speed of a complete frame or a sub-game must typically be 10 Hz or faster and the temperature range should be between 350xc2x0 C. and several 1000xc2x0 C.
U.S. Pat. No. 4,413,324 (Tatsuwaki et al.) describes an imaging pyrometer that makes use of an image pickup device whose pixels are covered with a mosaic of two types of infrared transmissive filters. The signals of two neighboring pixels are converted into a temperature value as in conventional two-wavelength pyrometers. This imaging pyrometer is capable of acquiring a two-dimensional temperature map of a scene. However, no provisions are foreseen to accommodate a large range of temperatures, requiring an unusually large dynamic range of the filtered pixels. Additionally, the temperature map is the only pictorial information acquired, and no visual image of the scene can be taken, for example in the visible spectral range, as is often required in machine vision for high-resolution optical inspection.
International publication No. WO-99/27336 (Koltunov et al.) describes an extension of U.S. Pat. No. 4,413,324 (Tatsuwaki et al.). The number of different types of infrared transmissive filter in a pixel is increased from 2 to N, where N is a natural number. This makes it possible to determine two-dimensional maps not only of the temperature but also the emissivity. As in U.S. Pat. No. 4,413,324 (Tatsuwaki et al.), no provision is foreseen to measure anything other than the temperature and emissivity map, or to accommodate a large range of temperatures requiring a usually large dynamic range of the filtered pixel in the filter mosaic.
U.S. Pat. No. 4,687,344 (Liliquist et al.) describes an imaging pyrometer that partially overcomes the limitations of Tatsuwaki et al. This device also consists of a single image pickup device, but it is completely covered with a single infrared transmission filter. Depending on the temperature range of interest, additional neutral density filters can be inserted to increase the effective dynamic range and therefore the temperature measurement range of the imaging pyrometer. However, since only one type of infrared filter is used, no two-wavelength correction can be made for surface properties of the emitting objects such as varying emissivity or surface finish, potentially leading to incorrect temperature readings. As in Tatsuwaki et al., only a temperature map is produced without any visible image.
U.S. Pat. No. 5,337,081 (Kamiya et al.) describes a triple-view imaging pyrometer, making use of a single image pickup device. The radiation incident from a scene is separated in two or more wavelength bands, and the resulting images of different wavelength bands are imaged onto different areas of the same single image pickup device. In this way, the registration problem is solved that exists when several different image pickup devices are used as described in previous patents. In this way, one can for example simultaneously acquire two-wavelength data for the calculation of the temperature map, as well as a visible image of the scene. However, the necessary optics to achieve the wavelength band separation and image combination for a single image pickup device is not trivial.
U.S. Pat. No. 5,963,311 (Craig et al.) describes an imaging pyrometer that makes use of a single image pickup device. In similar fashion as in Kamiya et al., beam splitting arid image combination optics is used to separate two wavelength ranges in the incoming radiation and to combine the two images onto one single image pickup device. The disclosed optical arrangement assures that the two images are in good geometrical registration on the image sensor. The two images are then used for conventional two-wavelength pyrometric determination of the temperature trap. However, the required optics is not trivial, and only a temperature map is acquired in this method.
It is an object of the present invention to provide an imaging pyrometer and a method for measuring surface temperature distributions of remote objects that overcome the above limitations of the prior art. More particularly, the invention shall solve the following two major problems:
(a) Two images of a scene shall be measured simultaneously and in perfect geometric registration: a reliable temperature map, based on the two-wavelength pyrometric measurement technique, and a high-resolution picture of the scene, for example in the visible spectral range.
(b) The dynamic range for the temperature map measurement and the simultaneous picture acquisition shall be increased compared to the prior art, so that both images are acquired tinder favorable signal-to-noise conditions, and the measurable temperature range between about 350xc2x0 C. and several 1000xc2x0 C. is accessible without additional neutral density filters or aperture stops.
The basic idea of the invention is to arrange at least three types of pixels for sensing electromagnetic radiation in at least three different spectral ranges in a mosaic pattern. This pattern has the following properties. In a neighborhood, there are two types of pixels with relatively narrow spectral sensitivity ranges in the infrared (IR), a first one (L) for sensing longer IR wavelengths and the other one (S) for shorter IR wavelengths. Additionally, there is a third pixel type (V) present for receiving electromagnetic radiation in a spectral range which is different from the sensitivity ranges of the first (L) and second (S) pixel types. (xe2x80x9cDifferentxe2x80x9d means in this connection that there are wavelengths in one spectral range that are not contained in the other spectral range.) This third pixel type V has the property of being densely arranged and regularly spaced, so that high-resolution, finely sampled images of the scene can be measured through these pixels, without being influenced by the measurements of the IR-sensitive pixels L, S. The sensitivity range of the third pixel type (V) is preferably adapted to the illumination of the scene to be imaged. Preferably, it is broader than the first (L) and second (S) sensitivity range, e.g., at least three times broader, and typically lies within or covers the visible part of the electromagnetic spectrum. Alternatively, it also may be relatively narrow, e.g., for cases where the scene is illuminated by a narrow-band light source such as a light emitting diode (LED).
In a preferred embodiment of the pyrometer according to the invention, a mosaic filter pattern is placed directly on pixels of an appropriate optoelectronic image sensor, for example by evaporation and photolithographic definition. Such color filters are well known in the art and may be, e.g., dielectric layer stacks, dye filters and/or diffractive filters (cf. K. Knop. xe2x80x9cColor Pictures Using the Zero Diffraction Order of Phase Grating Structuresxe2x80x9d, Optics Communications. Vol. 18, No. 3, 298-303, 1976). The first and the second type of IR-sensitive pixels L, S are related to two different types of IR transmission filters, a first one transmitting longer IR wavelengths, with a maximum transmission towards the longest wavelength where the image sensor is still sensitive, the other one at shorter IR wavelengths. The third type of pixels V can be related to a third type of filters transmitting in the visible part of the electromagnetic spectrum, or it could yield maximum sensitivity by avoiding a filter deposition on the corresponding pixels.
As an alternative, one can also use more than one imaging, filter type V1 . . . Vn, each of which has a different central wavelength and possibly a different spectral width.
To increase the dynamic radiometric and temperature range, and to adapt the signal levels of the differently sensitive pixels (L, S and V), the exposure times are adjusted to the actual radiation levels: if the temperature is high, then a relatively short exposure time is required to bring the L and S signals to a sufficiently high level. At 1000xc2x0 C., for example, exposure times of a few hundred microseconds are sufficient for a typical silicon-based CCD or CMOS image sensor. At 400xc2x0 C., however, exposures of several tens of milliseconds are required to reach the same signal level. Depending on the (additional) illumination level, the optimum exposure for the V pixels might be higher or lower than the one for the L and S pixels.
Depending on the type and capabilities of the solid-state image sensor employed, different means are available to adapt the exposure tine to the brightness level of the radiation:
(i) In a charge-coupled device (CCD) image sensor (of frame-transfer, interline-transfer or field-interline,transfer architecture), all pixels have the same exposure time. One can employ, therefore, two or more complete image acquisition cycles, each with its own exposure time. Two different exposure times are required when S and L pixels show similar signal levels and only V has a different signal level. Three exposure times are required when S, L and V pixels show all different signal levels. More than three exposure times are appropriate when S, L and V1 all show different signal levels, in the case of more than one V channel.
(ii) In a complementary-metaloxide-semiconductor (CMOS) image sensor or an active-pixel sensor (APS), it is possible to have a different exposure time for each imager line, for each column or even for each pixel. With such an image sensor type, it is not necessary to acquire three different images: it is rather possible to give each pixel type (S, L or V) its optimum exposure time, depending on temperature and illumination conditions. Since CMOS or APS image sensors offer non-destructive readout, this can be accomplished, for example, by reading out pixels which need short exposure times earlier than pixels with a long exposure time, relative to a reset signal given to the pixels.
The imaging pyrometer according to the invention has, among others, the following possible applications:
Laser welding. For controlling the laser power that must be high initially, until the surface of the object starts to become liquid and increases its absorbance suddenly, when the laser power must be reduced within a few 100 xcexcs so as not to evaporate the material but rather keep it in the liquid phase.
Laser drilling and cutting. For controlling the intensity of the laser used in drilling and cutting processes, and for advancing the object at the optimum speed. This can be done by observing the temperature around the cutting position.
Gas or electric welding. For controlling the speed at which an object advances under a gas or electric welding torch. The temperature is a measure for the optimum speed with which the welding can proceed.
Hot-air fusion of plastic materials. Plastic materials can be fused (xe2x80x9cweldedxe2x80x9d) by hot-air guns. In order not to burn the plastics, it is important to control the temperature of the hot air and the advancement of the materials to be welded. By observing the temperature map across the width of the hot air, dangerous hot spots and potential burns can be detected early and avoided.
Combustion process monitoring. The imaging pyrometer can observe simultaneously the burning pattern of a flame (xe2x80x9chow it dancesxe2x80x9d) and the combustion temperature. This information can be used to optimize the combustion process for lower pollutant emission and higher burner efficiency.
Combustion motor optimization. By observing at the same time the surface temperatures of parts in an internal combustion motor (e.g., a Diesel engine) and the precise motion of the different motor elements, the geometry of the engine and details of the combustion process can be optimized.
Monitoring of heat treatment. Various industrial processes call for a heat treatment that often needs to be uniform. An example is the so-called rapid thermal annealing (RTA) employed in the semiconductor industry. Since this is realized with an array of lamps, active lamp control and RTA optimization with the disclosed imaging pyrometer is possible.
Glass, metal and ceramic processing. Industrial processes that make use of the controlled melting, forming and cooling of substances (such as glass, metal or ceramics) sometimes result in products that have internal stress and are therefore prone to failure because the cooling process was not uniform and slow enough. Closed-loop control of such melting-forming-cooling processes is possible with the imaging pyrometer.